diff --git a/Sources/AsyncHTTPClient/HTTPHandler.swift b/Sources/AsyncHTTPClient/HTTPHandler.swift
index 95bdaeaed..432f0ff11 100644
--- a/Sources/AsyncHTTPClient/HTTPHandler.swift
+++ b/Sources/AsyncHTTPClient/HTTPHandler.swift
@@ -32,14 +32,15 @@ extension HTTPClient {
         /// A streaming uploader.
         ///
         /// ``StreamWriter`` abstracts
-        public struct StreamWriter {
-            let closure: (IOData) -> EventLoopFuture<Void>
+        public struct StreamWriter: Sendable {
+            let closure: @Sendable (IOData) -> EventLoopFuture<Void>
 
             /// Create new ``HTTPClient/Body/StreamWriter``
             ///
             /// - parameters:
             ///     - closure: function that will be called to write actual bytes to the channel.
-            public init(closure: @escaping (IOData) -> EventLoopFuture<Void>) {
+            @preconcurrency
+            public init(closure: @escaping @Sendable (IOData) -> EventLoopFuture<Void>) {
                 self.closure = closure
             }
 
@@ -55,8 +56,8 @@ extension HTTPClient {
             func writeChunks<Bytes: Collection>(
                 of bytes: Bytes,
                 maxChunkSize: Int
-            ) -> EventLoopFuture<Void> where Bytes.Element == UInt8 {
-                // `StreamWriter` is has design issues, for example
+            ) -> EventLoopFuture<Void> where Bytes.Element == UInt8, Bytes: Sendable {
+                // `StreamWriter` has design issues, for example
                 // - https://github.com/swift-server/async-http-client/issues/194
                 // - https://github.com/swift-server/async-http-client/issues/264
                 // - We're not told the EventLoop the task runs on and the user is free to return whatever EL they
@@ -66,49 +67,52 @@ extension HTTPClient {
                 typealias Iterator = EnumeratedSequence<ChunksOfCountCollection<Bytes>>.Iterator
                 typealias Chunk = (offset: Int, element: ChunksOfCountCollection<Bytes>.Element)
 
-                func makeIteratorAndFirstChunk(
-                    bytes: Bytes
-                ) -> (
-                    iterator: NIOLockedValueBox<Iterator>,
-                    chunk: Chunk
-                )? {
-                    var iterator = bytes.chunks(ofCount: maxChunkSize).enumerated().makeIterator()
-                    guard let chunk = iterator.next() else {
-                        return nil
+                // HACK (again, we're not told the right EventLoop): Let's write 0 bytes to make the user tell us...
+                return self.write(.byteBuffer(ByteBuffer())).flatMapWithEventLoop { (_, loop) in
+                    func makeIteratorAndFirstChunk(
+                        bytes: Bytes
+                    ) -> (iterator: Iterator, chunk: Chunk)? {
+                        var iterator = bytes.chunks(ofCount: maxChunkSize).enumerated().makeIterator()
+                        guard let chunk = iterator.next() else {
+                            return nil
+                        }
+
+                        return (iterator, chunk)
                     }
 
-                    return (NIOLockedValueBox(iterator), chunk)
-                }
-
-                guard let (iterator, chunk) = makeIteratorAndFirstChunk(bytes: bytes) else {
-                    return self.write(IOData.byteBuffer(.init()))
-                }
+                    guard let iteratorAndChunk = makeIteratorAndFirstChunk(bytes: bytes) else {
+                        return loop.makeSucceededVoidFuture()
+                    }
 
-                @Sendable  // can't use closure here as we recursively call ourselves which closures can't do
-                func writeNextChunk(_ chunk: Chunk, allDone: EventLoopPromise<Void>) {
-                    if let nextElement = iterator.withLockedValue({ $0.next() }) {
-                        self.write(.byteBuffer(ByteBuffer(bytes: chunk.element))).map {
-                            let index = nextElement.offset
-                            if (index + 1) % 4 == 0 {
-                                // Let's not stack-overflow if the futures insta-complete which they at least in HTTP/2
-                                // mode.
-                                // Also, we must frequently return to the EventLoop because we may get the pause signal
-                                // from another thread. If we fail to do that promptly, we may balloon our body chunks
-                                // into memory.
-                                allDone.futureResult.eventLoop.execute {
-                                    writeNextChunk(nextElement, allDone: allDone)
+                    var iterator = iteratorAndChunk.0
+                    let chunk = iteratorAndChunk.1
+
+                    // can't use closure here as we recursively call ourselves which closures can't do
+                    func writeNextChunk(_ chunk: Chunk, allDone: EventLoopPromise<Void>) {
+                        let loop = allDone.futureResult.eventLoop
+                        loop.assertInEventLoop()
+
+                        if let (index, element) = iterator.next() {
+                            self.write(.byteBuffer(ByteBuffer(bytes: chunk.element))).hop(to: loop).assumeIsolated().map
+                            {
+                                if (index + 1) % 4 == 0 {
+                                    // Let's not stack-overflow if the futures insta-complete which they at least in HTTP/2
+                                    // mode.
+                                    // Also, we must frequently return to the EventLoop because we may get the pause signal
+                                    // from another thread. If we fail to do that promptly, we may balloon our body chunks
+                                    // into memory.
+                                    allDone.futureResult.eventLoop.assumeIsolated().execute {
+                                        writeNextChunk((offset: index, element: element), allDone: allDone)
+                                    }
+                                } else {
+                                    writeNextChunk((offset: index, element: element), allDone: allDone)
                                 }
-                            } else {
-                                writeNextChunk(nextElement, allDone: allDone)
-                            }
-                        }.cascadeFailure(to: allDone)
-                    } else {
-                        self.write(.byteBuffer(ByteBuffer(bytes: chunk.element))).cascade(to: allDone)
+                            }.nonisolated().cascadeFailure(to: allDone)
+                        } else {
+                            self.write(.byteBuffer(ByteBuffer(bytes: chunk.element))).cascade(to: allDone)
+                        }
                     }
-                }
 
-                // HACK (again, we're not told the right EventLoop): Let's write 0 bytes to make the user tell us...
-                return self.write(.byteBuffer(ByteBuffer())).flatMapWithEventLoop { (_, loop) in
                     let allDone = loop.makePromise(of: Void.self)
                     writeNextChunk(chunk, allDone: allDone)
                     return allDone.futureResult